1. Field of the Invention
This invention relates to a catalyst for use in preparing .alpha.,.beta.-unsaturated aldehydes. More particularly, the invention relates to the preparation of a catalyst characterized in that a chelating agent or a macromolecular chelating agent, or a reaction product of a chelating agent and any of the catalyst metals having the form of a chelate compound or macromolecular chelate compound, is added during the catalyst preparation stage. Especially, this invention provides a catalyst having a highly enhanced activity for producing .alpha.,.beta.-unsaturated aldehydes by gas phase catalytic oxidation of .alpha.,.beta.-unsaturated hydrocarbons.
2. Description of the Prior Art
It is well known in the art that metal oxide catalysts comprising as main metal components molybdenum, bismuth and a transition metal, especially iron, cobalt or nickel, and optionally containing other elements incorporated according to need, can be used for the gas phase catalytic oxidation of .alpha.,.beta.-unsaturated hydrocarbons to produce .alpha.,.beta.-unsaturated aldehydes. Conventional catalysts of this type, however, have various defects.
These known catalysts include at least 5 metals as constitutuent metals and they can contain more than 10 metals in some cases. Accordingly, the catalyst system is very complicated and the preparation of such catalysts involves various problems and difficulties. There is a risk that reproducible catalytic activity cannot be obtained. Moreover, in many cases, expensive metals or toxic substances are used in the catalyst.
For example, an oxide catalyst of the Mo-Fe-Bi-As system disclosed in Japanese Patent Publication No 4771/69 and an oxide catalyst of the Mo-Bi-Co-Ni-Fe-As system disclosed in Japanese Patent Publication No. 6246/69 contain a poisonous substance, such as arsenic oxide, as a component for improving the yield of the .alpha.,.beta.-unsaturated aldehyde product. Therefore, these catalysts involve a risk that a poisonous substance, such as arsenic, can be discharged outside the reaction system, under some reaction conditions or depending on the working process. What is more important is that since arsenic compounds are very toxic and harmful, there is a danger that workers will suffer serious injury during preparation of arsenic-containing catalysts or during handling of such catalysts, for example, when withdrawing them from the reaction vessel. Accordingly, a process for preparing .alpha.,.beta.-unsaturated aldehydes that uses a catalyst containing an arsenic compound cannot be regarded as an industrially optimum process.
Catalysts comprising as main components, for example, oxides of Mo, Bi and Fe, are generally prepared by a method comprising dissolving ammonium molybdate in water, separately forming an aqueous solution of ferric nitrate and bismuth nitrate acidified with nitric acid, mixing this acidified aqueous solution with the above solution of ammonium molybdate, adding a suitable carrier and other salts to the liquid mixture according to need, and then conducting the conventional concentration, drying, molding and calcination steps to prepare the final catalyst composition.
The thus-prepared oxide catalysts of the Mo-Bi-Fe system have a lower catalytic activity than the oxide catalysts of the Mo-Bi-Fe-As system disclosed in Japanese Patent Publication No. 4771/69. It is difficult to produce .alpha.,.beta.-unsaturated aldehydes on an industrial and economically advantageous scale using such oxide catalysts of the Mo-Bi-Fe system. Further, in preparing such catalysts, molybdenum reacts with iron to form a gel-like precipitate, thereby degrading the dispersed state of the metals and resulting in a difficulty of obtaining reproducible results in the preparation of the catalysts. Moreover, the formation of the gel-like precipitate during the catalyst preparation stage causes a reduction of the catalytic activity of the resulting catalysts.